This invention relates to a method and apparatus for measuring properties (e.g., thickness) of thin layers (e.g., metal films) contained in a structure.
During fabrication of microelectronic devices, thin films of metals and metal alloys are deposited on silicon wafers and used as electrical conductors, adhesion-promoting layers, and diffusion barriers. Microprocessors, for example, use metal films of copper, tungsten, and aluminum as electrical conductors and interconnects; titanium and tantalum as adhesion-promoting layers; and titanium:nitride and tantalum:nitride as diffusion barriers. Thickness variations in these films can modify their electrical and mechanical properties, thereby affecting the performance of the microprocessor. The target thickness values of metal films vary on their function: conductors and interconnects are typically 3000-10000 angstroms thick, while adhesion-promoting and diffusion-barrier layers are typically between 100-500 angstroms thick.
During fabrication of the microprocessor, films are deposited to have a thickness of within a few percent of their target value. Because of these rigid tolerances, film thickness is often measured as a quality-control parameter during and/or after the microprocessor""s fabrication. Noncontact, nondestructive measurement techniques (e.g., optical techniques) are preferred because they can measure patterned xe2x80x9cproductxe2x80x9d samples, rather than xe2x80x9cmonitorxe2x80x9d samples. Measurement of product samples accurately indicates errors in fabrication processes and additionally reduces costs associated with monitor samples.
Optical methods for measuring thin, opaque films have been described. For example, U.S. Pat. No. 5,633,711 (entitled MEASUREMENT OF MATERIAL PROPERTIES WITH OPTICALLY INDUCED PHONONS), U.S. Pat. No. 5,546,811 (entitle OPTICAL MEASUREMENT OF STRESS IN THIN FILM SAMPLES), U.S. Pat. No. 5,672,330 (entitled MEASURING ANISOTROPIC MATERIALS IN THIN FILMS), and U.S. Ser. No. 08/783,046 (entitled METHOD AND DEVICE FOR MEASURING THE THICKNESS OF OPAQUE AND TRANSPARENT FILMS) describe an optical measurement technique called impulsive stimulated thermal scattering (xe2x80x9cISTSxe2x80x9d). In ISTS two optical pulses are overlapped on a sample to form a spatially and temporally varying excitation pattern that launches counter-propagating acoustic waves. These patents and applications have the same inventors as this application, and are incorporated herein by reference. U.S. Pat. No. 5,394,413 (entitled PASSIVELY Q-SWITCHED PICOSECOND MICROLASERS) describes a small-scale xe2x80x9cmicrolaserxe2x80x9d that can be used to form the excitation pulses. U.S. Pat. No. 5,734,470 (entitled DEVICE AND METHOD FOR TIME-RESOLVED OPTICAL MEASUREMENTS) describes how a single pulse passes through a diffractive mask, e.g. a phase mask, to form the two optical pulses. These patents are also incorporated herein by reference.
In ISTS the acoustic waves a xe2x80x9ctransient gratingxe2x80x9d that includes an alternating series of peaks and nulls. A probe pulse irradiates the grating, and is diffracted to form a pair of signal beams. One or both of the signal beams are detected and analyzed to measure a property of the sample.
In another embodiment of ISTS, the two excitation beams are separated from a single beam by a partially reflecting mirror (e.g., a beamsplitter) and used to form the transient grating. The probe beam is then focused on a peak or null of the grating, where it is reflected, detected, and analyzed to determine a property of the sample; in this case the diffracted beam is not detected. Accuracy in this measurement requires the phase of the grating to be fixed, i.e., the position of the peaks and nulls must be stationary relative to the probe beam. The peaks and nulls are typically separated by a few microns, and thus even small fluctuations of the laser beam causes these components to move relative to the probe beam; over short periods of time this averages out any modulation (e.g., signal) mapped onto the probe beam. Since beams generated by conventional lasers typically undergo spatial fluctuations, active stabilization systems including optical detectors, closed-loop feedback systems, and electrooptic light modulators (or similar means) are typically used in the measurement. Such systems fix the position of the peaks and nulls, making it possible to accurately measure the reflected beam.
U.S. Pat. No. 4,522,510 describes another optical technique wherein a single excitation beam irradiates a sample and is absorbed to initiate a xe2x80x9cthermal wavexe2x80x9d. A probe beam reflects off the thermal wave and is analyzed to determine a property (e.g., concentration of implanted ions) of the sample.
In general, in one aspect, the invention provides both a method and apparatus that when used a xe2x80x9creflection modexe2x80x9d geometry measure a property of a structure that includes at least one layer. The apparatus features a laser (e.g., a microchip laser, described below) that generates an optical pulse, and a diffractive element that receives the optical pulse and diffracts it to generate at least two excitation pulses. An optical system, (e.g., an achromat lens pair) receives the optical pulses and spatially and temporally overlaps them on or in the structure to form an excitation pattern that launches an acoustic wave (or an electronic response, or a thermal response). The acoustic wave modulates a property of the structure, e.g., it generates a time-dependent xe2x80x9csurface ripplexe2x80x9d or modulates an optical property such as the sample""s refractive index or absorption coefficient. Surface ripple is defined as a time-dependent change in the morphology of the surface; its peak-to-null amplitude is typically a few angstroms or less. The apparatus also includes a light source that produces a probe beam that reflects off an area of the structure containing the modulated property to generate a signal beam. An optical detection system receives the reflected signal beam and in response generates a light-induced electrical signal. An analyzer analyzes the signal to measure the property of the structure.
In embodiments, the diffractive element is a phase mask that includes an optically transparent substrate (e.g., a quartz plate or microscope slide). The substrate typically features one or more patterns characterized by a series of parallel trenches having a spatial periodicity of between 0.1 and 100 microns.
As described above, the laser is typically a microchip laser that is diode-pumped and passively Q-switched. For example, the laser can include Nd:YAG, titanium:sapphire, chromium:LISAF, analogs of these materials, or a fiber laser. In typical embodiments the laser features a Nd:YAG layer having a thickness of less than 5 mm. The laser used in the apparatus emits a pulse having a duration that is typically 1 nanosecond or less.
The acoustic waves typically modulate a structure""s surface. When the acoustic waves generate a time-dependent ripple on the surface the probe beam is aligned to deflect off the ripple to form the signal beam. In this case, the optical detection system includes a detector (e.g., a bi-cell detector or photodiode, described below) that generates an electrical signal that changes with a deflection angle of the probe beam. Alternatively, the modulated property is an optical property of the structure, such as a refractive index or absorption coefficient. Here, the probe beam reflects off an area of the structure containing the modulated optical property, and the optical detection system is configured to detect a phase of the reflected signal beam. Here, the optical detection system includes an interferometer.
In other embodiments, the optical system includes at least one lens that collects and overlaps the excitation pulses on or in the structure. For example, the optical system can include a lens pair (e.g., an achromat pair) having a magnification ratio of about 1:1. xe2x80x9cAbout 1:1xe2x80x9d means between 0.8:1 and 1.2:1. The apparatus also typically includes a lens that focuses the probe laser beam onto the acoustic waves. For example, when the acoustic waves generate a time-dependent ripple morphology, the probe beam irradiates a peak, null, a region between a peak or null, or a portion thereof. The probe beam can also irradiate a region of the sample undergoing a time-dependent change in refractive index or absorption coefficient.
The analyzer is configured to determine a frequency or phase velocity of the acoustic waves. These parameters can be used to determine a thickness of one or more layers in the structure. For example, the analyzer can be configured to calculate a thickness of the layer by analyzing the frequency or phase velocity, a density of the layer, and a wavelength of the excitation pattern. In other embodiments, the analyzer is configured to determine the density, resistivity, adhesion, delamination, elasticity, roughness, or reflectivity of the structure. Details of this calculation are included in the above-described patents and patent applications previously incorporated herein by reference.
The structure typically includes a silicon wafer, and the layer is a metal film composed of aluminum, tungsten, copper, titanium, tantalum, titanium:nitride, tantalum:nitride, gold, silver, platinum, or alloys thereof.
In another aspect, the apparatus features a passively Q-switched laser chat generates an optical pulse, and a photodiode that receives a portion of the optical pulse to generate a trigger pulse. The other components of the apparatus are similar to those described above. In this case, the apparatus includes a data-acquisition system that receives a light-induced electrical signal (from, e.g., a photodetector) and the trigger pulse. In response, the system generates a data signal (e.g., a signal waveform, described below) that is analyzed to measure a thickness of the layer in the structure.
In another aspect, the apparatus described above measures a property from a structure that does not contain a thin film. For example, the structure can be a semiconductor, such as an ion-implanted silicon wafer. The property can be an energy or concentration ion the ions implanted in the wafer.
Both the method and apparatus described above have many advantages. For example, these inventions use an all-optical measurement technique that effectively measures the thickness of thin films in multilayer structures without having to contact the sample. The thickness values can then be used to control a fabrication process (e.g., fabrication of a microelectronic device). The apparatus features all the advantages of optical metrology: each measurements is noncontact, rapid (typically less than 2), remote (the optical system can be as far as 10 cm from the sample), and can be made over a small region (as small as about 20 microns). Data collected in this way are analyzed to determine, e.g., film thickness with an accuracy and repeatability of a few angstroms.
Data collected in the reflection-mode geometry are optimized when the peaks and nulls of the grating are stationary relative to the focused probe beam. Two primary components for stabilizing the grating are described below: i) a diffractive mask that images the grating onto the sample is used in place of a partially reflecting optic (e.g., a beamsplitter); and ii) a small-scale microchip laser that generates a spatially stable beam is used in place of a conventional laser. These components, taken alone or combined, increase the signal-to-noise ratio of data collected in the reflection mode. This, in turn, improves the precision and accuracy to which a sample is measured.
Use of diffractive optics (e.g., a phase mask) in the optical system has particular advantages, especially when the reflected probe beam is measured and then analyzed to determine film thickness. In this embodiment, the probe beam is typically focused to a spot that is smaller than the width of a peaks and nulls in the optically induced grating. For example, the spot can be located on a grating peak, a grating null, or and area between neighboring peaks and nulls. The sample reflects or transmits the probe beam, which is then measured to determine: i) beam deflection; ii) a change in reflectivity or transmittivity due to the acoustic waves or other light-induced process; and/or iii) the optical phase or amplitude of the reflected or transmitted beam.
A pattern of a diffractive mask is imaged onto the sample to form the excitation pattern. This means that the peaks and nulls of the transient grating are spatially stabilized over long periods of time, even despite spatial fluctuations in the position or direction of the laser beam that irradiates diffracting mask. No closed-loop feedback system to stabilize the grating""s phase is required. The degree to which the probe beam is modulated can be optimized by moving the probe beam across the grating pattern or by translating the diffracting mask to a pattern that moves the position of the peaks and nulls.
In addition, pulses separated by a phase mask produce significantly stronger signal beams compared to those generated by pulses separated by conventional beam-splitting methods relying on beamsplitters. This is because pulses leaving the phase mask have parallel xe2x80x9cphase frontsxe2x80x9d, while pulses separated by a partially reflecting mirror have angled phase fronts. When overlapped in a sample, the parallel phase fronts produce many more light regions than the angle phase fronts. This decreases the damping of the acoustic waves, thereby increasing the precision to which it can be measured. This advantage is described in more detail in U.S. Ser. No. 09/086,975 (entitled METHOD AND DEVICE FOR MEASURING THE THICKNESS OF THIN FILMS IN MULTILAYER STRUCTURES, filed on the same day as the present application), the contents of which are incorporated herein by reference.
Use of a small-scale microchip laser in the reflection-mode geometry also minimizes spatial fluctuations of the grating, thereby increasing the quality of the measured data. This is because the beam emitted from the microchip laser undergoes very small spatial fluctuations, presumably due to the laser""s compact geometry. A typical microchip lasers features thin layers of a gain media (typically Nd:YAG) bonded to a saturable absorber (typically Cr+4YAG) The total size of this structure is typically less than 1 mm3. During operation, the microchip laser is pumped by an external diode laser. Because of its small size, the microchip laser is heated uniformly during the pumping process and requires only air cooling. These factors significantly reduce the amount of xe2x80x9cPointing instabilitiesxe2x80x9d (i.e. spatial beam fluctuations) typically present in larger lasers. This results in a very stable beam that has basically no spatial jitter, and consequently increases the spatial stability of the excitation pattern. Another advantage is that use of a diffractive mask and an imaging system minimizes the number of optical elements (e.g., mirrors, beamsplitters) required to form an excitation pattern on a sample""s surface. Only a single lens pair is needed. This simple optical system potentially reduces spatial fluctuations between the excitation pattern and the probing area caused by other optics (e.g., mirrors and beamsplitters), thereby allowing signal waveforms having high signal-to-noise ratios to be acquired.
Other features, aspects, and advantages of the invention follow from the following detailed description, and from the claims.